This invention relates generally to radar altimeter operations and more specifically, to a radar altimeter which includes an automatically calibrated sensitivity range control (SRC) function.
A radar altimeter typically includes a transmitter for applying pulses of electromagnetic energy, at a radio frequency (RF), and at regular intervals to a transmit antenna which then radiates the energy, in the form of a transmit beam, towards the earth's surface. A transmit beam from a radar is sometimes said to “illuminate” an area (e.g. the ground) which reflects (returns) the transmit beam. The reflected beam, sometimes referred to as a ground return, is received at a receive antenna of the radar altimeter. A signal from the receive antenna is processed to determine an altitude of the aircraft incorporating the radar altimeter.
However, radar altimeters typically provide marginal performance at very low altitudes, for example, during landing and take-off. During landing and take-off is when accurate operation of radar altimeters is most desired, at least in part, due to the proximity of the aircraft to the ground. The reduction in performance is partially due to interfering signals resulting from a leakage path between the transmit and receive antennas of the radar altimeter. In normal radar altimeter operations, as described above, a transmit antenna transmits a signal towards the ground which reflects the signal. The receive antenna receives the ground reflected signal for processing to determine aircraft altitude. A leakage path exists when a portion of the transmitted signal is directly received by the receive antenna without having been reflected by the ground.
Aircraft radar altimeter installations utilize separate transmit and receive antennas to enable a tracking of altitude down to a zero altitude. As described above, the ground return signal at low altitudes, for example, during landing and take-off operations, is difficult to process. The difficulty is partially caused by the leakage path signals. At low altitudes, the ground return signal corresponding to a transmitted signal is received, at a close proximity in time, to reception of the leakage path signal resulting from the same transmission. To counteract the leakage path signals, a gain of the receiver of the radar altimeter is typically decreased at low altitudes. Lowering the gain allows the radar altimeter to acquire the desired ground return signals rather than the leakage path signals. The gain is typically decreased as a function of decreasing altitude and this function is commonly referred to as sensitivity-range-control (SRC). Utilizing SRC, the gain of the radar altimeter receiver is reduced to a level to assure the receiver will not acquire and track the leakage path signals. As described, SRC is an open loop control, and a significant desensitization margin is added to SRC, typically greater than 10 decibels, to allow for antenna installation variances and radar altimeter receiver sensitivity variances.
During production, and as part of incorporating a sensitivity-range-control, known radar altimeters are calibrated for each different aircraft antenna installation. Further, these radar altimeters are temperature compensated to assure that sensitivity of the radar altimeter is lower than an expected antenna leakage signal, but also to retain as high of a sensitivity as possible to provide the best possible low altitude performance. Still further, the significant desensitization margin described above (e.g., 10 dB), is incorporated into radar altimeters to allow for leakage path signal amplitude variances. Such variances are caused by, for example, transmit and receive antenna separation distance variations, or variations in landing gear or munitions positions in the vicinity of the leakage path.